Generally, the use of operational amplifiers is common in circuit design. Traditionally, an operational amplifier (op-amp) comprised a single transistor pair with one of the differential inputs coupled to a single transistor. These single transistor pair op-amps generally could only operate with a common mode voltage VCM that had a magnitude such that the gate-to-source voltage VGS of each transistor was greater than the threshold voltage Vt. Any magnitude of the common mode voltage VCM that did not meet this condition caused the op-amp to be in an inoperable mode. Thus, these op-amps could not be operated at any common-mode voltage VCM between the power source rails, or in other words, could not operate rail-to-rail.
As the voltage of power sources decreased, the need to operate an op-amp rail-to-rail generally increased. In response to this need, complementary transistor pairs op-amps were introduced that comprised both a p-channel field effect transistor (PFET) pair and an n-channel field effect transistor (NFET) pair. These op-amps allowed rail-to-rail operation, but because the operation ranges of the PFET pair and the NFET pair overlapped in only a portion of the rail-to-rail common mode voltage VCM range, the transconductance of the op-amp GM was not generally constant. For example, when only the PFET pair was conducting, the transconductance GM was much less than the transconductance GM when both the PFET pair and NFET pair were conducting. This non-constant transconductance GM introduced problems in some circuit designs, for example, for frequency compensation for negative feedback.
Subsequent op-amps attempted to address this non-constant transconductance GM in complementary transistor pairs op-amps by steering the current through one of the transistor pairs when the other pair was not conducting to increase the transconductance GM during that operation. One way this was accomplished was by using current-mirror circuits. As a transistor pair became non-conducting, current from the pair's constant current source would be diverted into a current-mirror. The output of the current-mirror would be coupled in parallel with the other transistor pair's constant current source such that as current was diverted into the current-mirror, the current flowing through the other transistor pair would be increased. This typically would increase the transconductance GM of the op-amp while only one pair was conducting.
However, these prior art attempts to create a constant transconductance generally cause the current that flows in the op-amps to be greatly increased. This causes a much larger power consumption. Accordingly, there is a need in the art for a configuration for a complementary transistor pairs op-amp that realizes both a constant transconductance GM and lower current flows.